(CNN) — Six Chinese Communist Party investigators have been sentenced to between four and 14 years in jail after they drowned a local official during a botched interrogation that has shone light on China’s secretive extrajudicial processes.

Cheng Wenjie, the deputy director of the Commission for Discipline Inspection in the eastern city of Wenzhou, received the longest sentence of the six for his role in the death of Yu Qiyi, according to a copy of the court verdict given to CNN by the victim’s wife.

All the defendants pleaded guilty.

Yu, 41, the chief engineer at a state-owned company in the same city, died at a local hospital on April 9, a month after being detained over his role in a land deal.

After early interrogations failed to yield any admission of guilt, Cheng summoned other investigators April 8 and ordered Yu to strip naked and sit in a bathtub filled with icy water.

At Cheng’s behest, three other investigators– Li Xiang, Nan Yu and Gu Chenfu — grabbed his neck, arms and legs and held him under the water. The two other defendants — Wu Zhiwei and Zhang Fangchao — entered the room later the same evening, the verdict said.

When Cheng left he told his subordinates: “I’m leaving now; you all have fun with it.”

Yu was sent to hospital an hour later and died at 3am the next day, the verdict, which has not been officially released, said.

The court said that the six interrogators attempted to drown Yu in order to force him to admit that he had violated the law.

Yu’s wife, Wu Qian, told CNN the sentences handed down were not long enough, adding that many important facts about the case had not been aired in court.

She said she wanted to plan for her late husband’s funeral but the Commission for Discipline Inspection had not returned his remains.

Wu Pengbin, a lawyer acting on behalf of the deceased man, said that he had heard the six defendants had appealed their sentences as they believed they were too harsh.

Yu was placed under investigation as part of a secretive process known as shanggui or dual designation that allows for Communist Party members to be detained and interrogated by Party investigators.

It is based on the Party’s own regulations rather than laws and procedures, making it even less transparent than China’s judicial system.

Legal experts view the prosecution of Yu’s interrogators as a step forward but say that the key to preventing similar abuses is to address the root cause of his death – the rarely publicized shanggui system.

“The six investigators arrested were mere scapegoats,” Pu Zhiqiang, one of the lawyers representing the Yu family, told CNN in September. “Shuanggui is an illegal process — abuse and torture during interrogation have become standard operating procedure.”